Overall, yes, security has improved and even if it's a bit flaky at some airports it's much more of a deterrent than was the case prior to 11 Sept.
So the opportunity cost for a terrorist act involving aircraft is now much higher and I suspect the focus of terrorist groups will have moved to other areas; whatshouldiuse mentioned commuter trains. And of course ports - just imagine the damage a nuclear device on board a ship could do if detonated entering a large port. The US are tackling that one now, at immense cost.